Dynamics of charge currents ballistically injected in GaAs by quantum interference

The dynamics of charge currents ballistically injected in GaAs bulk and quantum wells are spatially and temporally resolved. The electrons and holes are injected with oppositely directed velocities without the use of accelerating fields by quantum interference between two photon absorption of a 200 fs, 1430 nm fundamental pulse and one photon absorption of the corresponding second harmonic pulse. The subsequent charge motion is followed with similar to 200 fs temporal and similar to 1 nm spatial resolution by using tightly focused optical differential transmission techniques that are dependent on the relative phase of the incident pump pulses. Initially, the electrons and holes ballistically separate by up to similar to 20 nm, and a space charge field forms, which decelerates the carriers. Within this similar to 1 ps regime, the momentum relaxes by electron-hole and phonon scatterings, and the space charge field restores the electrons and holes to a common position; on time scales long compared to 1 ps, ambipolar diffusion and recombination complete the return of the system to equilibrium. A rigid shift (damped simple harmonic oscillator) model for the electron motion reproduces the key features in the data, and the procedure for extracting the spatiotemporal dynamics of the electrons is shown to be immune to energy relaxation effects and forgiving of nonlinear saturation. (c) 2008 American Institute of Physics.